I am struggling with a problem like this: In dimension $n\geq 3$, consider the following uniformly elliptic equation

$$a^{ij}(x)u_{ij}(x)+u_{nn}=0$$

where $i,j = 1, \dots, n-1$, and $\lambda \operatorname{id} < (a^{ij}) < \Lambda \operatorname{id}$. Are there any interior gradient estimates? More specifically, I need the estimate $|u_n|\leq C$ in the interior, where $C$ depends only on $|u|_{L^\infty}$ and the elliptic constant.